1. Field of the Invention
This invention relates to an image display apparatus having a flat shape, and more particularly, to an image display apparatus provided with a number of electron emitting elements in a vacuum envelope and a manufacturing method and a manufacturing apparatus for the image display apparatus.
2. Description of the Related Art
Recently, various flat display apparatuses have been developed as a next generation of lightweight, thin image display apparatuses to replace cathode-ray tubes (hereinafter referred to as CRT). These flat display apparatuses include a liquid crystal display (hereinafter referred to as LCD), plasma display panel (hereinafter referred to as PDP), field emission display (hereinafter referred to as FED), surface-conduction electron emission display (hereinafter referred to as SED), etc. In the LCD, the intensity of light is controlled by utilizing the orientation of a liquid crystal. In the PDP, phosphors are caused to glow by ultraviolet rays that are produced by plasma discharge. In the FED, phosphors are caused to glow by electron beams that are emitted from field-emission electron emitting elements. In the SED, phosphors are caused to glow by electron beams that are emitted from surface-conduction electron emitting elements.
In general, the FED or SED, for example, has a front substrate and a rear substrate that are opposed to each other with a given gap between them. These substrates have their respective peripheral portions bonded together by means of a sidewall in the form of a rectangular frame, thereby constituting a vacuum envelope. A phosphor screen is formed on the inner surface of the front substrate. A number of electron emitting elements (hereinafter referred to as emitters) for use as sources of electron emission for exciting the phosphors to luminescence are provided on the inner surface of the rear substrate. In order to support atmospheric load that acts on the front substrate and the rear substrate, a plurality of support members are arranged between the substrates. The potential on the rear substrate side is substantially equal to the earth potential, and an anode voltage Va is applied to the phosphor screen. Electron beams that are emitted from the emitters are applied to red, green, and blue phosphors that constitute the phosphor screen, whereupon the phosphor layers are caused to glow, thereby displaying an image.
According to the FED or SED constructed in this manner, the thickness of the apparatus can be reduced to several millimeters. Therefore, the FED or SED can be made thinner and lighter in weight than a CRT that is used as a display of an existing TV set or computer.
In the FED or SED described above, moreover, a high vacuum must be formed in the envelope. Also in the PDP, the envelope must be evacuated before it is loaded with discharge gas.
As means for evacuating the envelope, there is a method in which the front substrate, rear substrate, and sidewall that constitute the envelope are heated and joined together by a suitable sealing material in the atmosphere. After the envelope is then exhausted through an exhaust pipe that is attached to the front or rear substrate, in this method, the exhaust pipe is vacuum-sealed. In the case of a flat envelope, however, the exhaust through the exhaust pipe is very slow, and the attainable degree of vacuum is low. Thus, the mass-productivity and properties are not reliable.
In another method, the front substrate and the rear substrate that constitute the envelope may be finally assembled in a vacuum tank. In this method, the front substrate and the rear substrate that are first brought into the vacuum tank are fully heated in advance. This is done in order to reduce the gas discharge from the inner wall of the envelope that constitutes the principal cause of lowering of the degree of vacuum. When the front substrate and the rear substrate are then cooled so that the degree of vacuum in the vacuum tank is fully improved, a getter film for improving and maintaining the degree of vacuum of the envelope is formed on the phosphor screen. Thereafter, the front substrate and the rear substrate are heated again to a temperature high enough to melt the sealing material. The front substrate and the rear substrate are combined together in a predetermined position as they are cooled so that the sealing material is solidified.
For the vacuum envelope constructed by this method, a sealing process doubles as a vacuum-sealing process. Besides, a lot of time that is required by the exhaust through the exhaust pipe can be saved, and a high degree of vacuum can be obtained.
In this assembly in a vacuum, however, processing in the sealing process involves various operations, such as heating, position alignment, and cooling, and the front substrate and the rear substrate must be kept in the predetermined position for a long period of time before the sealing material is melted and solidified. Since the front substrate and the rear substrate undergo thermal expansion as they are heated and cooled in the sealing operation, moreover, the alignment accuracy easily lowers. Thus, the sealing operation entails problems on productivity and properties.